Mild cosmetic compositions must satisfy a number of criteria including cleansing power, foaming properties and mildness/low irritancy/good feel with respect to the skin, hair and the ocular mucosae. Skin is made up of several layers of cells which coat and protect the keratin and collagen fibrous proteins that form the skeleton of its structure. The outermost of these layers, referred to as the stratum corneum, is known to be composed of 250 .ANG. protein bundles surrounded by 80 .ANG. thick layers. Hair similarly has a protective outer coating enclosing the hair fibre which is called the cuticle. Anionic surfactants can penetrate the stratum corneum membrane and the cuticle and, by delipidization destroy membrane integrity. This interference with skin and hair protective membranes can lead to a rough skin feel and eye irritation and may eventually permit the surfactant to interact with the keratin and hair proteins creating irritation and loss of barrier and water retention functions.
Ideal cosmetic cleansers should cleanse the skin or hair gently, without defatting and/or drying the hair and skin and without irritating the ocular mucosae or leaving skin taut after frequent use. Most lathering soaps, shower and bath products, shampoos and bars fail in this respect.
Certain synthetic surfactants are known to be mild. However, a major drawback of most mild synthetic surfactant systems when formulated for shampooing or personal cleansing is poor lather performance compared to the highest shampoo and bar soap standards. Thus, surfactants that are among the mildest are marginal in lather. The use of known high sudsing anionic surfactants such as alkyl sulphates with lather boosters, on the other hand, can yield acceptable lather volume and quality but at the expense of clinical skin mildness. These two facts make the selection of suitable surfactants in the lather and mildness benefit formulation process a delicate balancing act.
In addition to the mildness, cleansing and lathering performance attributes desired by consumers it is important that personal cleansing products further have good thickening and rheological properties, in product storage, in dispensing and in-use.
It is known that water-soluble polymers can be used to provide desirable product thickening attributes in surfactant systems and, furthermore, that hydrophobic modification of water-soluble polymers can improve their thickening efficacy. However, it is also known that the thickening properties of such hydrophobically modified water-soluble polymers can be significantly reduced in non-dilute, water-soluble surfactant systems (as discussed in Sau and Landoll `Polymers in Aqueous Media`, Advances in Chemistry Series No. 223, Chapters 8, 17, 18, edited by J. E. Glass). In particular, Applicant has found that in non-dilute, water-soluble surfactant systems, hydrophobically modified polymers, such as hydrophobically modified hydroxy ethyl cellulose, do not deliver good product thickening. Non-dilute, as defined herein, means systems comprising greater than about 1% by weight of water-soluble surfactant. Water-soluble, as defined herein, means a surfactant having a molecular weight of less than about 20,000 wherein the surfactant is capable of forming a clear, isotropic solution when dissolved in water at 0.2% w/w under ambient conditions (25.degree. C.).
It is also well known that fatty alcohol ethoxylates and fatty alcohols can thicken systems containing both water-soluble surfactant and electrolyte (as illustrated, for example, in Hoechst Surfactants, published Trade Literature). However, fatty alcohol ethoxylates and fatty alcohols modify the micellar structure of the water-soluble surfactant system, resulting in undesirable effects on product characteristics such as the lather profile, rinsing behaviour and in-use product feel attributes. Applicant has also found, that, in order to achieve good product thickening in non-dilute, water-soluble surfactant systems via the exclusive use of a fatty alcohol ethoxylate and/or fatty alcohol thickening agent, the amount of fatty thickener required to deliver acceptable thickening attributes results in reduced lather and rinsing performance.
It is known that an additional difficulty associated with the use of fatty alcohol ethoxylate and/or fatty alcohol and electrolyte based thickening systems is that the thickening effect of such materials is highly dependant upon the purity and quality of the raw materials used. This can lead to unpredictability in thickening performance, such as thin products with non-recoverable low viscosity. This unpredictability makes it difficult to efficiently formulate systems using water-soluble surfactants which have predictable viscosity profiles when using these fatty materials.
Thus a need exists for effective thickening systems for non-dilute, water-soluble surfactant systems which deliver good product thickening and rheology attributes both in storage, in dispensing and in-use in combination with the delivery of excellent product characteristics such as lather, cleanisng, rinsing, skin mildness and in-use skin feel attributres.
Surprisingly, it has now been found that personal cleansing compositions having excellent product thickening and rheology attributes, both under product storage and in-use conditions are provided by the combination of a thickening system consisting essentially of associative polymer and polar oil in combination with a non-dilute, water-soluble surfactant system. Furthermore it has also been found that the thickening system of the present invention can deliver desirable thickening effects from a significantly lower level of total thickener than previously utilised in surfactant compositions.
In addition to the product thinning difficulties faced when attempting to thicken systems containing high levels of surfactant using fatty thickeners, applicant has found that mild, water-soluble nonionic surfactants, such as polyhydroxy fatty acid amides and alkyl polyglycosides are difficult to thicken using fatty thickeners and electrolyte based thickening systems.
Applicant has also found that the thickening system of the present invention is particularly valuable for thickening certain mild water-soluble nonionic surfactants such as polyhydroxy fatty acid amide surfactant.